Question

1. If there are 6500 degree-days in this area, and the price of fuel is $10 per million Btu, then find the heating cost per square foot per heating season for the un-insulated floor. (ans: $0.68/ft²)
2. If 6-in fiberglass costs $0.40 per square foot, what will be the payback time (as a result of energy savings) on this installation? (ans: 0.7 seasons)

Answer 1

a)

$\frac{Q}{A}=\frac{1}{R}\times 24 hrs/day\times DD_{annual}=\frac{1}{2.3}\times 24\times 6500=67826.1Btu/ft^{2}$

Heating Cost

$=67826.1\times \frac{10}{10^{6}}$

=$0.678/ft² =$ 0.68 /ft²

b)

$\frac{Q}{A}=\frac{1}{R}\times 24 hrs/day\times DD_{annual}=\frac{1}{19}\times 24\times 6500=8210.5263 Btu/ft^{2}$

Savings per season=0.68-8210.5263*(10/10⁶)=$ 0.596 /season

Payback time =0.4/0.596 =0.671 seasons =0.7 seasons (approx)